gl/cocoa: Return a strong ref to the parent GstGLContext
If the GstGLCAOpenGLLayer was initialized via a parent context, make sure to ref the context before returning it from getGLContext as all callers will unref it.
Follow up to !3729 (merged)
If the GstGLCAOpenGLLayer was initialized via a parent context, make sure to ref the context before returning it from getGLContext as all callers will unref it.
Follow up to !3729 (merged)